Salt peptized proteinaceous adhesive and coating composition employing the same as abinder



SALT PEPTEZED PROTEHNACEOUS ADHESIVE AND QOATING COMPOSITION EMPLOYINGTHE AME AS A BINBER Ralph l Mahaney, llelcit, Wis, assignor to S. D.Warren Company, Boston, Mass a corporation of Massachusetts No Drawing.Filed Nov. 9, 1959, er. No. 851,541

4 tllaims. (Cl. 106-146) This invention relates to proteinaceousadhesives and more particularly to a process for peptizing suchadhesives in an acid medium and to a mineral filled coating compositionemploying the same as a binder precipitated therein in the free state.

One of the problems presented in drum finishing paper by the well knowncast coating process has to do with the water solubility of the adhesivebinder in the coating. Experience has demonstrated that it ispractically impossible to cast a high gloss surface it the adhesivebinder in the coating has been water insolubilized and dried prior tothe casting step. On the other hand, the adhesive should be waterinsolubilized at some point in the process or else the finished productwill be too readily affected by water during use. One solution for theproblem has been to coat the paper prior to casting it, and, then as thepaper enters the casting nip, to introduce insolubilizing chemicals intothe re-wetting solution at the casting nip. Such a process has beensuccessful in part, but one of the primary disadvantages is that theliquid in the casting nip washes oil a substantial part of the coatingand the part washed off cannot be recovered economically. Accordingly,it is one of the objects of this invention to provide an adhesive binderfor mineral coating compositions which, in combination with mineralfillers, etc. will cast well even though the coating has been previouslydried and is in a relatively Water insoluble state. Another object is toprovide such an adhesive and mineral coating composition which will, atone and the same time, cast well and not wash ofi to any appreciableamount when subjected to a re-wetting bath at the casting nip.

Another problem in the art of coating relates more specifically to thedisadvantages of calendering. In order to accomplish any appreciabledegree of smoothing of conventional coating surfaces by calenderingwithout at the same time crushing the body stock, the paper must bedried in order to give the fibers in the body stock enough strength towithstand the pressure of the calender roll. In general, it is adequateto air-dry the paper (about 4% moisture) for this purpose, and when thisis done a calender roll pressure of approximately 2,000 pounds perlinear inch will smooth the coating surface adequately without crushingthe body stock. On the other hand, due to the extreme pressure requiredwith conventional coating materials, calendering has previously alwayslowered the bulk, brightness and opacity of the paper, and otherwisegiven the paper a somewhat dingy appearance. Accordingly, for many yearsnow, the paper industry has sought a process for smoothing a coatedpaper surface to the same extent as may be accomplished by calenderingwhile at the same time preserving the bulk, opacity and brightness ofthe uncalendered body stock. Thus, in addition to providing a coatingmaterial having special advantages in cast coating, it is also an objectof our invention to provide a coating material which can be smoothedunder relatively light roll pressure Without substantial loss in bulk,opacity and/ or brightness, and which is also water insoluble.

In the accomplishment of these and other objects in a preferredembodiment of my invention, I formulate a coating composition comprisingconventional coating components in combination with a proteinaceousadhesive.

3,081,182 Patented Mar. 12, 1963 My invention depends in part on mydiscovery that casein, or casein-like materials such as soy-beanprotein, may be peptized in a concentrated solution of the casein,together with a salt such as ammonium nitrate, sodium nitrate, ornitrates of other alkali metals. In order for this to take place theconcentration of casein-to-water must be quite high; i.e. 2035%.Likewise the concentration of salt must be comparatively high; i.e.4l0%. When the casein, salt and water are intermixed in thesepercentages and heated to approximately ISO-200 F. and subjectedtovigorous agitation, the casein is peptized or appears to dissolve inapproximately 20 minutes. The addition of a small amount (below 10%based on casein) of water-soluble tallow-based soap assists thepeptizing process and permits some lowering of these percentages.

Obviously, the addition of the soap must not be so great beforedescribing how the casein is combined with the clay,

a few general remarks about the characteristics of the casein sopeptized will help to form a basis for understanding the nature of theadhesive. If it is allowed to cool to room temperature, it rapidly gelsto a thick mass. Reheating the mass, however, returns the casein to itsformer liquid state. On the other hand, if the liquid, peptized casein,at for instance F., is poured into an excess of water (no matter whetherthe water is hot or cold) instead of forming a thinner solution ofpeptized casein, the casein immediately precipitates and forms aconglomerate ropelike mass. Likewise if water is poured into such caseinwithout agitation, the same sort of precipitation takes place, and evenwith agitation, precipitation seems to occur. The ropy cords are notformed, but the solution turns milky in appearance. In addition, sincecasein hy-;

drolyzes if allowed to remain at elevated temperature, we generallyprefer to lower the temperature and agitation of the solution shortlyafter the casein has been peptized, and likewise we prefer at that timeto combine it with the other coating ingredients to minimize the efiectsof hydrolysis.

Turning now to the combination of my salt peptized casein with othercoating ingredients, this is done by forming a slurry of clay and waterand dumping an appropriate amount of the clay slurry into the saltpeptized casein while continuously agitating the mixture. the sharp dropin temperature, together with the dilution of the adhesive by the excessof water in the slurry immediately causes the casein to gel and/orprecipitate and also fiocculates the clay slurry. This causes the entireslurry now mixed with adhesive to become quite stiff and hard to stir.As the agitation is continued, however, apparently the casein, eventhough somewhat precipitated, tends to disperse the clay and the mixturegradually starts to loosen up. In practice after the agitation hascontinued for a few minutes, the mixture suddenly thins out and becomesquite fluid. I am not sure just what happens between the casein and theclay to bring about this result, but the thinning out of the coatingmixture is very marked and readily observed.

The resulting combination of salt peptized casein and clay has a numberof interesting properties. In the first place, it has a surprisingly lowviscosity which makes it an excellent binder for coatings employed withan air-knife smoothing mechanism. In addition, the Water retention ofthe fluidcoating composition prior to coating is very low and the rateof water absorption of the finished dried coating is high. Anothercharacteristic of the comtogether they appear to be far more malleablethan ordi- When this is done,.

nary coating compositions. Thus under conditions of heat, controlledmoisture content, and light pressure, the coating may be smoothed bycalender rolls without at the same time substantially reducing the bulk,brightness and/or capacity of the coated sheet. Along with the foregoingcharacteristics an extremely important characteristic relates to castcoating. The casein in my coating combination seems to be largelyprecipitated and, therefore, it is substantially water insoluble.However, even though water insoluble, it is still in a condition whichpermits drum finishing by the cast coating process while at the sametime strongly resistingany tendency to wash off when passed through are-wetting bath at the casting nip during the casting operation.

Several specific examples of the process of my invention will now bedescribed.

EXAMPLE NO. 1

'In this example the following ingredients were mixed in the following,proportions'of parts'by weight:

180'parts water (at 180 F.)

60 parts casein- (Australian hydrochloric acid precipitated) 12- partsammoniumnitrate (technical grade fertilizer sold-under the trademarkAeropril 12 parts dicyandiamide (soldunderthe trademark Azite) part soap(tallow based).

The foregoing mixture had a-pH- of about 5.0. After agitating the samefor about 20 minutes during whichthe temperature gradually declined toabout 140 F., the

solution became quite clear and had a slightly yellowish hue. In thiscondition, it is considered that the casein is substantially peptized.However, it should be noted'that' the salt employed to peptize thecasein does not combine with-the casein to form a caseinate as do thealkalisnormally employed to disperse casein.

Withthe casein in this peptized condition and at a temperature of about140 F., a slurry containing approximately 400 parts clay and 400 partswater at room temperature was dumped into the casein and simultaneouslystrongly agitated. Immediately following the combination of these twoingredients, the slurry became stiff and viscous and the agitationpropeller commenced laboring heavily. For 2 or 3 minutes the agitationcontinued with great difficulty and thereafter the slurry commenced tothin out in the neighborhood of the propeller, and thence rapidly'tobecome entirely quite free and liquid. At this point care had to betaken in the control'of the agitator to prevent a sudden spraying forthof the contents of the'mixing' container as the ingredients loosened up.

In this condition the material was ready for coating, and while it isbelieved that the casein was precipitated, yet the-mixture wascomparatively free and without lumps. On. the other hand, when theseingredients were coated on paper and observed microscopically, theappearance was" substantially more of a pebbly nature and not as smoothin appearance as alkaline dispersed casein. How-- EXAMPLE NO. 2

In this example the same ingredients as Example No. 1 were employed,except that both the soap and the dicyandiamide were omitted. The resultof this was to make it more diflicult to peptize the casein, but withcontinuous vigorous agitation, the casein did peptize in about 30minutes.

EXAMPLE NO.. 3

In. this example the same procedures as set' forth for Example No-1 werefollowed, except that the proportion of soap wasincreased to:approximately 1% This mix-' ture peptized slightly quicker, and afterbeing combined with the clay slurry, it resulted in a coating which wasparticularly well suited for hot calendering under light pressure.

EXAMPLE NO. 4

In this example the following ingredients were mixed in the followingproportions of parts by weight:

parts water (at 180 F.)

60 parts casein (Australian hydrochloric acid precipitated) 12 partssodium nitrate.

In the foregoing mixture the casein peptized in about 30 minutes underconditions of continuous heat and vigorous agitation.

Theoretical Discussion It is believed that this novel method forpeptizing'casein results from an ionic interaction between the salt andthe casein in the concentrations'and under the conditions set forth.Taking, for instance, the illustration of Example No. 1, in solution theammonium nitrate divides into positively charged ammonium ions andnegatively charged nitrate ions. Apparently these oppositely chargedions'in the concentrations shown, and at or near the isoelectric'pointof the'casein, have'the power to neutralize the electrostatic attractiveforces of the Zwitter ions of the casein molecule, with the positivelycharged ammonium'ions of the salt neutralizing the negatively chargedca'rboxyl and carbonyl groups of'the casein, and the negatively chargednitrate ions of the salt neutralizing the positively charged amine orimide groups of the casein.

The foregoing discussion is important in the context of my inventionbecause it explains how the casein can be peptized eventhough it is ator near its iso-electric point. Also ithelps lead to a selection ofvarious other salts which can serve in the process. In general, the saltshould be a stable, soluble, neutral or acidic salt of an alkali metalor ammonium having a monovalent cation and a monovalent anion. As apractical matter the nitrates of ammonium and sodium are preferred.

In general the pH of the salts which perform satisfactorily in ourprocess have in a 5% aqueous solution a pH of 4.5 to 7.0. If salts whichare more acidic are employed, they fail to peptize the casein and ifthey are more basic, they tend to yield a caseinate similar to theconventional alkaline casein dispersants.

The foregoing discussion also explains why the casein peptized by themechanism of my invention precipitates upon the addition of Water andthe dilution of the concentration of the salt; If the dilution iscarried out with an ammonium nitrate solution of similar concentrationin place of the Water, this precipitation does not occur.

Numerous minor variations of my invention will now be apparent to thoseskilled in the art, and therefore, it is not my intention to confine theinvention to the precise form herein shown but rather to limit it interms of the appended claims. Moreover, in connection withthe'combination' of coating materials set forth, there is no need for meto set forth the many coating combinations wherein my casein may beemployed, since a great variety thereof are well known to those skilledin the art. Thus in place of clay, I may employ calcium carbonate, blancfixe, talc, titanium dioxide, and the like. Anti-foaming and releaseagents may also be employed, as Well as combinations of latex or otherpolymeric substances for the purpose of imparting known and desiredproperties.

When I employ the term salt peptized proteinaceous adhesive in theclaims of this application, I intend the term to include broadly suchadhesives peptized at a pH relatively close to their iso-electric pointby a nitrate of ammonium or alkali metal in accordance with the stepsset forth above. In addition, when I employ the term unalteredproteinaceous adhesive or unaltered casein,

I intend to describe the proteinaceous adhesive peptized in accordancewith the process of this invention, it being understood that saltpeptizing does not involve a chemical change of forming a caseinarte asdo the conventional processes involving the use of bases to disperse theadhesive.

Having thus described and disclosed preferred embodiments of myinvention, what I claim as new and desire to secure by Letters Patent ofthe United States of America is:

1. A process for preparing a coating composition that yields a maleablecoating on a paper web comprising: preparing a peptized aqueous solutionof a protein selected from the group consisting of casein and soyprotein by admixing at a temperature in the range of 140 to 212 F. saidprotein with a soluble salt of a monovalent anion and a cation selectedfrom the group consisting of the alkali metal ions and the ammonium ionwhile maintaining the pH of the admixture between the iso-electro pointof said protein and 7, said soluble salt in a 5 percent aqueous solutionhaving a pH in the range of 4.5 to 7.0, said protein being in the rangeof 20 to 35 percent of said solution and said soluble salt being in therange of 4 to 10 percent thereof, thereafter forming a stiff slurry byadmixing said aqueous solution with a cool slurry 6 of apaper-coating-grade mineral pigment, the amount of said slurry beingsuflicient to dilute the concentration of said salt in the resultingmixture to below about 4 percent where it no longer peptizes saidprotein, and agitating said stiff slurry to a homogeneous mixture ofsubstantially lower viscosity.

2. The process of claim 1 when said soluble salt is selected from groupconsisting of sodium nitrate and ammonium nitrate.

3. The process of claim 1 when said aqueous solution also contains aviscosity reducing agent.

4. The coating composition produced by the process of claim 3.

References Cited in the file of this patent UNITED STATES PATENTS2,198,596 Atwood Apr. 30, 1940 2,214,564 Montgomery et a1 Sept. 10, 19402,334,607 Christopher Nov. 16, 1943 2,356,795 Poarch Aug. 29, 19442,360,828 Craig Oct. 24, 1944 2,367,678 Hatch et a1. Jan. 23, 19452,581,111 Landes et a1. Jan. 1, 1952 2,894,847 Wright July 14, 1959

1. A PROCESS FOR PREPARING A COATING COMPOSITION THAT YIELDS A MALEABLECOATING ON A PAPER WEB COMPRISING: PREPARING A PEPTIZED AQUEOUS SOLUTIONOF A PROTEIN BY FROM THE GROUP CONSISTING OF CASEIN AND SOY PROTEIN BYADMIXING AT A TEMPERATURE IN THE RANGE OF 140* TO 212* F. SAID PROTEINWITH A SOLUBLE SALT OF A MONOVALENT ANION AND A CATION SELECTED FROM THEGROUP CONSISTING OF THE ALKALI METAL IONS AND THE AMMONIUM ION WHILEMAINTAINING THE PH OF THE ADMIXTURE BETWEEN THE ISO-ELECTRO POINT OFSAID PROTEIN AND 7, SAID SOLUBLE SALT IN A 5 PERCENT AQUEOUS SOLUTIONHAVING A PH IN THE RANGE OF 4.5 TO 7.0, SAID PROTEIN BEING IN THE RANGEOF 20 TO TO 35 PERCENT OF SAID SOLUTION AND SAID SALT BEING IN THE RANGEOF 4 TO 10 PERCENT THEREOF, THEREAFTER FORMING A STIFF SLURRY BYADMIXING SAID AQUEOUS SOLUTION WITH A COOL SLURRY OF APAPER-COATING-GRADE MINERAL PIGMENT, THE AMOUNT OF SAID SLURRY BEINGSUFFICIENT TO DILUTE THE CONCENTRATION OF SAID SALT IN THE RESULTINGMIXTURE TO BELOW ABOUT 4 PERCENT WHERE IT NO LONGER PEPTIZES SAIDPROTEIN, AND AGITATING SAID STIFF SLURRY TO A HOMOGENOUS MIXTURE OFSUBSTANTIALLY LOWER VISCOSITY.